The invention relates to a method of cooling the continuous shielding wire fed to the welding rollers of machines used for seam-welding discrete lengths of tube. Such machines are provided with a pair of cooled mill rolls at a point upstream of the welding rollers, in relation to the direction of the wire feed, between which the continuous wire is passed and rolled flat.
Conventional machines used for welding the longitudinal seams of tin cans are provided with a horizontal arm, one end of which is in receipt of metal strip, either in continuous format or discrete lengths; the strip material is fed in at right angles to the arm and folded piece by piece into tubular shape, with the longitudinal edges overlapping.
The discrete lengths of tube are then conveyed along the arm toward its remaining end where two welding rollers, located opposite one another outside and inside of the embodied tube, proceed to seam-weld the two longitudinal edges together, the necessary heat being generated by passage of electric current between the rollers and the welded material (Joule effect).
In order to prevent inordinate wear on the welding rollers, and to avoid their being fouled excessively through contact with the tinned material from which the cans are fashioned, the rollers themselves are shielded by a continuous conductive wire that makes direct contact with the overlapping longitudinal edges of the folded length of tube and thus ensures that the welding rollers stay clean.
Before reaching the welding rollers, the continuous wire (initially circular in section) passes through a pair of cooled mill rolls that spread it in order to produce a flat cross section, thereby increasing the surface area by way of which contact is made with the longitudinal edges of the discrete length of tube and ensuring, clearly, that a faultless overlap is maintained at the weldment.
One problem which besets the use of the continuous wire shield is that of excessive heat generated in the wire itself during the welding process. This heat causes the wire to stretch inordinately, with the result that tension becomes difficult to control.
A method and apparatus designed to overcome the problem are disclosed in application for European Patent No. 122333, which claims the adoption of a drum or cylinder around which the wire is wrapped for cooling purposes, located immediately upstream of the first welding roller, and at a point between the first and second welding rollers.
Whilst a method of this type is able to bring about a marked reduction in the temperature of the wire, it obviously calls for utilization of an entirely separate piece of apparatus, which can be exploited solely for this express purpose.
Accordingly, the object of the invention disclosed is to eliminate the drawback mentioned above, in a simple and economical manner.